Date Published: June 12, 2019
Publisher: Public Library of Science
Author(s): Pierre Lau, Vaughn Bryant, James D. Ellis, Zachary Y. Huang, Joseph Sullivan, Daniel R. Schmehl, Ana R. Cabrera, Juliana Rangel, Wolfgang Blenau.
For honey bees (Apis mellifera), colony maintenance and growth are highly dependent on worker foragers obtaining sufficient resources from flowering plants year round. Despite the importance of floral diversity for proper bee nutrition, urban development has drastically altered resource availability and diversity for these important pollinators. Therefore, understanding the floral resources foraged by bees in urbanized areas is key to identifying and promoting plants that enhance colony health in those environments. In this study, we identified the pollen foraged by bees in four developed areas of the U.S., and explored whether there were spatial or temporal differences in the types of floral sources of pollen used by honey bees in these landscapes. To do this, pollen was collected every month for up to one year from colonies located in developed (urban and suburban) sites in California, Texas, Florida, and Michigan, except during months of pollen dearth or winter. Homogenized pollen samples were acetolyzed and identified microscopically to the lowest taxonomic level possible. Once identified, each pollen type was classified into a frequency category based on its overall relative abundance. Species richness and diversity indices were also calculated and compared across states and seasons. We identified up to 64 pollen types belonging to 39 plant families in one season (California). Species richness was highest in CA and lowest in TX, and was highest during spring in every state. In particular, “predominant” and “secondary” pollen types belonged to the families Arecaceae, Sapindaceae, Anacardiaceae, Apiaceae, Asteraceae, Brassicaceae, Fabaceae, Fagaceae, Lythraceae, Myrtaceae, Rhamnaceae, Rosaceae, Rutaceae, Saliaceae, and Ulmaceae. This study will help broaden our understanding of honey bee foraging ecology and nutrition in urban environments, and will help promote the use of plants that serve the dual purpose of providing aesthetic value and nutritious forage for honey bee colonies placed in developed landscapes.
Honey bee (Apis mellifera L.) workers dedicate most of their foraging phase specializing in the collection of nectar, pollen, propolis, or water . Floral nectar provides the carbohydrates needed for a colony’s energetic needs, while pollen, the main source of protein, provides bees with ten essential amino acids that are critical for brood rearing and queen feeding [2–5]. Pollen is conserved during storage by mixing it with nectar and glandular secretions from workers to create what is known as “bee bread” . Nurse bees consume bee bread to develop their hypopharyngeal glands, which produce a protein-rich jelly that is used to feed developing larvae [7, 8]. Although a colony typically demands more carbohydrates than proteins, pollen can often become a limiting nutritional factor due to low resource availability or quality at certain times of the year [5, 9, 10]. For instance, deficiencies in the availability of particular amino acids can create a bottleneck in brood rearing , and without adequate amounts and types of pollen, colonies can quickly deplete their protein reserves, leading to a reduction in brood rearing and even brood cannibalism .
We conducted a one-year survey of the floral sources of pollen foraged by managed honey bee colonies in urban and suburban environments in four different regions of the U.S. Using standard melissopalynological techniques, we identified pollen pellets collected by foragers in these environments at spatial and temporal scales. Overall, the “predominant” and “secondary” pollen sources collected by foragers across all states originated from plants belonging to various genera in the Fabaceae (legumes), Anacardiaceae (sumac), Lythraceae (loosestrife), Arecaceae (palm), Asteraceae (daisies and asters), Fagaceae (oak), Sapindaceae (soapberry), Rhamnaceae (buckthorn), Salicaceae (willow), Myrtaceae (eucalyptus), Rosaceae (rose), and Brassicaceae (mustard) families (S1 Table, S2 Table). There was also a large representation of pollen from trees and shrubs for colonies in TX, FL, and CA, including those in the families Lythraceae, Arecaceae, Fagaceae, Salicaceae, Myrtaceae, and Rosaceae. These types of trees and shrubs have previously been shown to be important pollen sources for pollinators in urbanized areas [65, 66]. Interestingly, the majority of pollen collected in MI in the summer and fall came from herbaceous plants rather than trees and shrubs, indicating the importance of herbs as honey bee forage in this region of the U.S.